The present proposal is concerned chiefly with defining the contribution of the 2 intestinal lecithin synthetic pathways which might be expected to provide the lecithin associated with intestinal lipoprotein formation and specifically that associated with chylomicrons. The first pathway, which is dependent on lysolecithin acyltransferase, was shown to supply all the chylomicron lecithin when 10 mM lecithin was infused with triglyceride into the duodenum of rats whose common bile duct and main intestinal lymphatic had been cannulated. By contrast, when 32P was infused to estimate the contribution of the CDP choline dependent pathway, incorporation into chylomicron lecithin was high at low lecithin infusate concentrations and approximated 0 at high (10 mM) lecithin infusion concentrations. During the coming year, the contribution of circulating lecithin to chylomicron lecithin synthesis will be studied by infusing (14C glycerol) lecithin and lysolecithin into the femoral vein of rats during triglyceride and lecithin infusion. The contribution of membrane, i.e. preformed lecithin, to chylomicron lecithin will be estimated in rats injected with (3H methyl) choline and subsequently infused with lipid or isocaloric glucose.